Are carbon fibres beneficial to improve mechanical and fracture properties of FDM printed composites?

Fused Deposition Modeling (FDM) is currently the most accessible additive manufacturing technology due to its affordability and the simplicity of its machinery, coupled with the low cost and widespread availability of thermoplastic polymers. In this study, the mechanical properties of virgin thermoplastic materials (PLA and PETG) under bending and dynamic tests, as well as the impact of their carbon fiber reinforcement are investigated. All samples were manufactured under the same conditions using the same FDM machine. A study is also made on the tensile properties of the material wires, studying their morphology in detail in order to highlight possible signs that would reveal the nature of the influence of fiber reinforcement on the overall mechanical behavior of the printed components. The experimental results revealed contrasting effects of carbon fiber reinforcement on PLA and PETG. PLA-CF exhibited microcrack formation (crazing), which led to increased ductility but also to a decrease in tensile strength, flexural strength, and fracture toughness, indicating a general embrittlement of the composite. In contrast, PETG-CF showed improved mechanical and fracture properties compared to plain PETG, most likely due to better interfacial adhesion between the fibers and the polymer matrix, although with a more brittle behavior.